The existence of the ferroelectric compound Sr.sub.x Ba.sub.1-x Nb.sub.2 O.sub.6 was first reported in 1960 see M. H. Francombe, Acta Crystallography 13, 131 (1960)]. The composition was found to fall within the range 0.20&lt;.times.&lt;0.70, and was said to be closely related in structure to the tetragonal tungsten bronze structure. In the mid 1960's, the first Sr.sub.X Ba.sub.1-x Nb.sub.2 O.sub.6 single crystals were grown over a range of compositions 0.25&lt;.times.&lt;0.75 [see A. A. Ballman and H. Brown, Journal of Crystal Growth 1, 311 (1966)]. These single crystals were found to possess large electro-optic coefficients, indicating their potential use in laser modulation and deflection devices. The favorable electrical properties of these single crystals has also led to their use in pyroelectric detection devices.
The vast majority of work in the SBN area has centered around the production of single crystal materials. These crystals are formed by a conventional technique, known as the Czochralski method, which involves growing the crystals from a molten bath containing the SBN constituents. U.S. Pat. No. 4,001,127 to Megumi et al describes such a process for making single crystal SBN, whereby a melt containing strontium oxide, barium oxide and niobium pentoxide is prepared, and a seed crystal is used to grow a single crystal of SBN from the melt. Megumi et al control the amounts of strontium oxide, barium oxide and niobium pentoxide present within the melt in order to obtain high quality SBN single crystals having improved homogeneity.
U.S. Pat. No. 4,187,109, to Megumi et al, relates to single crystal SBN which is doped with a transition metal such as cerium, vanadium or uranium. The single crystals are grown using the Czochralski method, and are said to be capable of photo-induced refractive index changes.
Studies have shown that single crystal Sr.sub.0.75 Ba.sub.0.25 Nb.sub.2 O.sub.6 (SBN 75) possesses superior optical sensitivity, which allows for its use as a holographic storage medium (see J. B. Thaxter and M. Kestigan, Applied Optics, Vol. 13, No. 4, pp. 913-924, 1974). Studies have also shown that single crystal Sr.sub.0.60 Ba.sub.0.40 Nb.sub.2 O.sub.6 (SBN 60) is suitable for use as a substrate material in the growth of thin films by the liquid phase epitaxial technique (see R. R. Neurgaonkar and E. T. Wu, Materials Research Bulletin, Vol. 22, pp. 1095-1102, 1987).
However, several difficulties are associated with the growth of single crystals, including the inability to grow large crystals, and the lack of homogeneity. Crystal sizes are typically limited in diameter to 2 or 3 centimeters, while "striation" along the growth axis and "core", normal to the growth axis, cause inhomogeneity. Additionally, the cost of growing single crystals is high.
Polycrystalline (ceramic) materials have a number of advantages over single crystal materials. They may be formed in virtually any size or shape, whereas single crystals are limited by crystallographic orientation and growth conditions. Ceramic materials, and in particular ferroelectric ceramics, are typically formed by the mixed oxide technique, which involves mixing oxide starting powders and forming the mixture into the desired shape, by such methods as pressing, sintering, and hot isostatic pressing (HIPing). Ceramics such as Al.sub.2 O.sub.3 -MgO, Y.sub.2 O.sub.3 -ThO.sub.2, and Pb.sub.1-3x/2 La.sub.x Zr.sub.1-y Ti.sub.y O.sub.3 (PLZT) have been made by the mixed oxide technique.
U.S. Pat. No. 4,019,915, to Miyauchi et al, relates to a mixed oxide method for making ferroelectric ceramic materials of the composition ABO.sub.3, wherein A includes Pb and at least one of the elements Ba and Sr, and B includes Zr or a combination of Zr and Ti.
Polycrystalline SBN has been produced by the mixed oxide method. Carruthers et al describe the phase equilibria relations in polycrystalline SBN produced by the mixed oxide technique (see J. R. Carruthers and M. Grasso, Journal of the Electrochemical Society: SOLID STATE SCIENCE, Vol. 117, No. 11, pp. 1426-1430, 1970). The method of preparation taught by Carruthers et al requires the use of high temperatures, on the order of 1400.degree. C. and higher, which are typical of the mixed oxide method.
In recent years, an alternative method known as sol-gel processing has been used to make ceramic materials. The sol-gel process begins with chemical precursors which may either be a solution or a colloidal dispersion (sol) of extremely fine particles. Chemical reactions, which cause polymerization in the continuous phase, are then initiated and the liquid sets to a rigid gel. The gel is then dried and fired to form a polycrystalline body. The sol-gel technique has been used to form a limited number of ceramics, including glasses, silica, and thin films. However, it is believed that the sol-gel method has not heretofore been used successfully to form SBN materials.